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doi:10.2204/iodp.proc.303306.107.2006

Physical properties

Physical property data at Site U1307 were measured on both whole-round sections and discrete samples from split-core sections. Whole cores were measured every 5 cm. Samples were collected from each core in Hole U1307A for moisture and density (MAD) property measurements.

Whole-core magnetic susceptibility measurements

Magnetic susceptibility records are highly variable, responding to lithologic and/or mineralogic changes. Magnetic susceptibility measurements were obtained from the Fast Track MSCL and from the multisensor track (MST) system. There is an offset in susceptibility values between the MSCL and the MST records (see “Physical properties” in the “Site U1302–U1308 methods” chapter), but both records show the same variability downcore.

The magnetic susceptibility record shows multiple excursions toward high values (Fig. F25), with an average magnetic susceptibility of ~411 × 10^–5 SI. No clear long-term trend is evident in the data, although there is some hint that the upper 10 m records lower magnetic susceptibility values. High-frequency variations occur at scales of a meter or less throughout the Site U1307 records. The amplitude of these meter-scale fluctuations is variable downcore. The largest peaks in magnetic susceptibility commonly correspond to darker layers (see “Lithostratigraphy”). Lower magnetic susceptibility values of 70 × 10^–5 to 280 × 10^–5 SI were observed and, in general, correspond to sediment of lighter color. An interval of exceptionally low magnetic susceptibility was recorded at 154–64 mcd (Cores 303-U1307A-18H and 303-1307B-16H). Here, values shifted abruptly to 27 × 10^–5 to 30 × 10^–5 SI (Fig. F26). This excursion to low values does not correlate with any major lithologic change (see “Lithostratigraphy”). X-ray diffraction (XRD) analysis of sediments immediately above and within this low-magnetic susceptibility interval show identical bulk mineralogic records.

Density

Gamma ray attenuation (GRA) density values record a general increase downcore from ~1.55 to ~1.76 g/cm³. Discrete bulk density values generally fall within the range of GRA density values, recording a similar downhole increase (Fig. F27). This density increase is typical of Expedition 303 sites and results from sediment compaction. Superimposed on this long-term increase are meter-scale fluctuations of ~0.2 g/cm³. In most cases, single-point peaks correspond to large clasts (>2 cm) in the core (see “Lithostratigraphy”).

Natural gamma radiation

NGR counts generally increase downcore (Fig. F28). Much of the downhole increase in NGR values appears to occur within two intervals: 50–60 mcd and 115–130 mcd (roughly corresponding to lithologic Unit II; ~50–134 mcd) with average NGR counts of 25.9 and 31.7 cps, respectively. Below 130 mcd, average NGR is 34.3 cps. A steep decrease in NGR values occurs below ~157 mcd in Core 303-U1307B-17H. This trend is not recorded in Hole U1307A; however, Core 303-U1307B-17H is disturbed, which may account for the discrepancy between Holes U1307A and U1307B.

P-wave velocity

P-wave velocities range from 1543 to 1604 m/s at the top of the sedimentary sequence (upper 80–85 mcd) (Fig. F29). Velocity records from the MST (P-wave logger [PWL]) and the split-core measurements from the P-wave sensor number 3 (PWS3) are discontinuous, which may be a function of gas content in the sediments. Signal strength deteriorated below ~85–90 mcd in Hole U1307A and below ~90 mcd in Hole U1307B, often generating readings with values lower than that of seawater. This deterioration coincides with a rapid downhole increase in methane at ~85 mcd (see “Geochemistry”). PWL and PWS3 measurements also show an offset in values throughout the entire section. This is apparently a long-standing "calibration" problem not unique to Expedition 303 (see “Physical properties” in the “Site U1302–U1308 methods” chapter).

Porosity

Porosity was calculated using MST GRA density measurements (see “Physical properties” in the “Site U1302–U1308 methods” chapter) and spot-checked with discrete pycnometer measurements (Fig. F30). The GRA density-derived porosity and discrete MAD measurements (Fig. F30) agree well. Although highly variable, porosity decreases from values of 65%–40% to ~42%–39% at the bottom of Hole U1307A. At the bottom of Hole U1307B, a steep increase in porosity to values >70% coincides with disturbed Core 303-U1307B-17H.

Discussion

Site U1307 was placed in an area where the Quaternary sedimentation rates were lower, thus allowing the older record to be recovered by APC coring. All three Eirik Drift sites (Sites U1305–U1307) have similar long-term NGR, density, and porosity records, whereas magnetic susceptibility values show distinct differences among the sites. The average magnetic susceptibility value at Site U1307 is 425 × 10^–5 SI. This average is very similar to the average value at Site U1305 (436 × 10^–5 SI), but much lower than the average value at Site U1306 (626 × 10^–5 SI). This is surprising because Site U1307 has a similar water depth and is located closer to Site U1306; hence, they might be expected to have similar depositional patterns.

The lowest range of magnetic susceptibility variation at Site U1307 appears to be recorded in the uppermost 10 m. Farther downhole, numerous high magnetic susceptibility peaks generally correspond to darker sediment layers, several of which contain large terrigenous particles (>2 cm), whereas lighter-color sediments are usually characterized by low magnetic susceptibility values. This is particularly evident in intervals characterized by high carbonate content such as nannofossil ooze layers (e.g., at 20, 24.5, and 147.7–148.5 mcd) and in the interval corresponding to gray foraminiferal sand (47.9–50 mcd). These foraminiferal sands are also characterized by low NGR values and high density, unlike most of the other carbonate-rich levels. There is an apparent increase in the high-frequency variability of magnetic susceptibility values from 110 to 125 mcd that may in part be related to an increase in the deposition of terrigenous-dominated sediments (see “Lithostratigraphy”), thus diluting the other sediment components. Hence, we suggest that the terrigenous versus carbonate content of the sediments is an important control on magnetic susceptibility values at Site U1307.

One of the most interesting features of the magnetic susceptibility record is an interval of exceptionally low magnetic susceptibility values between 154 and 164 mcd, whereas NGR shows little or no change in character. This interval is also generally characterized by low density values. Identical bulk mineralogic records were obtained from XRD within and outside the low-magnetic susceptibility interval. Postcruise analyses are required to fully understand the causes of the variability within the physical property records and to interpret them in the context of paleoclimate change.

At Site U1307, a high-frequency signal is evident in all of the physical properties. The NGR records covary with magnetic susceptibility records in some intervals, but in other intervals they are out of phase with magnetic susceptibility records. More detailed studies are required to determine the relationships among the sedimentary record and these forcing factors.

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